Fluorescent dyes are used for the purpose of visualizing a sample, and are widely used in the biological and biochemical fields as a labeling material (labeling dye), orthochromatic dye, recognition probe of biologically relevant substances, and for photodynamic therapy (PDT). Fluorescent dyes having absorbance and fluorescence in the long wavelength region, especially in near infrared region (particularly, 650 nm to 900 nm) can be measured even in the presence of biological substances such as body tissues, blood, lipids and water without being optically disturbed by these substances. Further, in view of the advantages such as low energy and high optical permeation, application thereof to the imaging of tissues at a deep part in the body is expected.
Fluorescent dyes having a long wavelength are used not only in the biological and biochemical fields, but also in chemical and other fields. For example, they are often used widely as a red display material or laser dye, or as an optical recording material. Thus, fluorescent dyes with a long wavelength are demanded not only in biological field but also in a wide variety of fields.
The properties demanded for an excellent fluorescent dye with a long wavelength, e.g., for a fluorescent dye for labeling biological substances include the following:
1) fluorescence with a long wavelength;
2) high molar extinction coefficient;
3) high fluorescence quantum yield;
4) sharp absorption spectrum;
5) unresponsiveness to environment (response to solvent is small); and
6) variety of wavelengths (dyes with various wavelengths can be synthesized).
At present, as a molecule satisfying a part of the above-mentioned conditions, boron dipyrromethene skeleton (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes) represented by the following structural formula is known:

Although boron dipyrromethene skeleton is an excellent dye having a high light fastness, high fluorescence quantum yield and sharp absorption spectrum, it does not have a sufficient function as a near infrared fluorescent dye because the fluorescence wavelength of the basic skeleton is about 500 nm. Thus, some researches have been conducted for making the wavelength of boron dipyrromethene longer. The methods therefor include 1) introduction of a strong electron donating group(s); 2) making the skeleton stiffer; and 3) extension of conjugated system.
As for 1), as represented by, for example, Patent Literature 1 or Non-patent Literature 1, the shift of the wavelength to the longer wavelength can be attained by introducing strong electron donating groups as R1 and R7. However, increase in the molar extinction coefficient has not been attained thereby. Further, since the stronger the electron donating properties of the introduced functional groups, the more likely the photo-induced electron transfer (PET) occurs, so that the decrease in the fluorescence quantum yield occurs and the dependence of the fluorescence quantum yield on the polarity of the solvent becomes stronger. More particularly, since the fluorescence quantum yield of these molecules are drastically decreased in a polar solvent such as water or methanol, they are not suited for biological analyses.
As for 2), as represented by, for example, Non-patent Literature 2, it has been reported that shift of wavelength to longer wavelength is attained by crosslinking the electron donating groups introduced as R1 and R7 with R2 and R6, respectively, through an appropriate methylene chain or a hetero atom. However, increase in the molar extinction coefficient has not been attained thereby, and the synthetic method is complicated so that variation of the compounds is limited.
The above-mentioned 3) is a method wherein the shift of wavelength to longer wavelength is attained by extending the conjugated double bond such as olefin as in cyanine dyes. However, as frequently reported for cyanine dyes, there is a concern that decrease in the photostability and fluorescence quantum yield due to photoisomerization of the olefin may occur, so that it is not an effective method. Further, although shift of the wavelength to longer wavelength can be attained by fusing aromatic rings with R2 and R3, and R5 and R6, respectively, increase in the molar extinction coefficient has not been attained thereby. Further, by the reported synthetic method, the ring which can be fused is limited to an aromatic ring, so that the variation of the dye is limited.    Patent Literature 1: U.S. Pat. No. 5,248,782 B    Patent Literature 2: U.S. Pat. No. 5,433,896 B    Patent Literature 3: U.S. Pat. No. 6,005,113 B    Non-patent Literature 1: A. Burghart et. al. J. Org. Chem. 1999, 64, 7813.    Non-patent Literature 2: J. Chen et. al. J. Org. Chem. 2000, 65, 2900.    Non-patent Literature 3: Current Medicinal Chemistry 2005, 12, 795-895.